In a conventional large-screen liquid crystal display, a light-diffusing plate is provided so as to eliminate the lamp image of a light source (e.g., a fluorescent tube) of a backlight unit.
The light-diffusing plate is required to have such properties as to diffuse light transmitted therethrough over a wide angular range with respect to a direction perpendicular to the plate.
In order to illuminate the entire surface of a liquid crystal unit with the thus-transmitted light at high brightness, generally, at least one prism sheet is provided between the liquid crystal unit and the light-diffusing plate, which serves as a support of the prism sheet.
The prism sheet is provided for convergence or diffusion of light within an angular range effective for increasing brightness.
However, in some cases, during the course of assembly of the backlight unit of the liquid crystal display, two prism sheets must be provided in combination with an additional functional film (e.g., light-diffusing film or retardation film), thereby requiring a prolonged and intricate process and resulting in an increase in production cost attributed to the prism sheets.
In view of the foregoing, for the purpose of elimination of such a prism sheet, demand has arisen for a prism-integrated light-diffusing plate which serves as both a functional film and a light-diffusing plate (thick plate having a thickness of 0.5 to 3 mmt).
There has been proposed a light-diffusing prism sheet produced by forming microprotrusions having a serrate cross section on at least one surface of a light-diffusing sheet containing a transparent resin (100 parts by mass) and cross-linked acrylic resin beads having a mean particle size of 1 to 20 μm (0.5 to 10 parts by mass) (see, for example, Patent Document 1).
Although this technique is proposed for the purpose of provision of a light-diffusing prism sheet serving as both a light-diffusing film and a prism sheet, the technique is not applied to a thick light-diffusing plate which is employed particularly in a large-screen liquid crystal TV (17 inches or more) and which serves as both a light-diffusing layer and a supporting layer.
As has been known, even when a prism pattern is transferred onto a conventional light-diffusing plate which is employed in a direct-type backlight, the thus-formed prismatic portion fails to exhibit light converging effect, due to high light diffusivity of the plate.
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 9-304606